allantois

[ə-lăn′tō-ĭs]

n.pl.al•lan•to•i•des (ăl′ən-tō′ĭ-dēz′)

A membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles, and is important in the formation of the umbilical cord and placenta in mammals.allantoid

allantois

[ə-lăn′tō-ĭs]

Plural allantoides(ăl′ən-tō′ĭ-dēz′)

A membranous sac that grows out of the lower end of the alimentary canal in embryos of reptiles, birds, and mammals. In mammals, the blood vessels of the allantois develop into the blood vessels of the umbilical cord.